In general, an air-cooled motor is cooled down with a number of cooling fins vertically provided on the circumference of a motor housing by making use of natural radiation or by blowing air from a forced-air cooling fan to the cooling fins. In either case, since heat is only absorbed from the motor with the medium of air contacting the surfaces of the cooling fins, satisfactory cooling effect is not expected, and besides, it is necessary to provide a sufficient space around the motor housing for feeding fresh air to the cooling fins at all times, resulting difficulty in arranging mechanical components close to each other.
When an air-cooled motor is used for driving a machine tool and cooling of the air-cooled motor is carried out by blowing air from the forced-air cooling fan to the cooling fins, air having absorbed heat from the motor is blown against a column or a feed shaft of the machine tool, with the result that variation in the dimensions of members due to thermal expansion may lower accuracy. For this reason, it is customary to set the blowing direction of air from the forced-air cooling fan to be parallel with a motor shaft for preventing such a problem from arising, and a number of machines are designed is such a manner.
However, spindle-through-coolant is often applied to the recent machine tool by directly connecting a main shaft to the motor shaft for the speed-up of the main shaft and the improvement in power efficiency. In this case, as shown in FIG. 4, for instance, a motor shaft 101 has a coolant feed passage (a through hole) 100, and a coolant feed pipe 102 and a pipe joint 103 are arranged on the axial rear side (that is, the side opposite to the main shaft, i.e., the right side in FIG. 4) of the motor shaft 101. On the other hand, the main shaft (not shown) is located on the axial front side (i. e., the side of the main shaft, i.e., the left side in FIG. 4) of the motor shaft 101. Thus, a forced-air cooling fan cannot be provided on either side i.e., the axial front side and the axial rear side of the motor shaft 101. For this reason, a forced-air cooling fan 105 is provided in the vicinity of the center of a motor housing 104 in such a posture that the axial center of a rotary shaft of the forced-air cooling fan is allowed to extend orthogonally to the axis of the motor shaft 101, as shown in FIG. 4.
In the prior art shown in FIG. 4, the center of the motor housing 104 and the forced-air cooling fan 105 are covered with a hood 106, and a motor 107 is forcedly air-cooled as if the motor takes the shape of a ducted fan. However, as only the center of the motor 107 is cooled, cooling efficiency of the motor is low, and if a column or a feed shaft is disposed in the course of warmed air, accuracy may be lowered.